Project Summary/Abstract Successful implementation of mother-to-child-transmission prevention programs has decreased perinatal HIV infections from 500,000 to 150,000 cases per year in the last decade. But it is now evident that this has created a large and expanding number of HIV-exposed but uninfected infants (HEU) who have more morbidity and mortality than their HIV-unexposed (HU) counterparts. Some estimate that over 1 million children per year, or 30% of all births in some areas, are HEU children. However, nearly all of the available evidence of these adverse clinical outcomes for HEU comes from less rigorous cross sectional or retrospective studies, and the few prospectively followed birth cohorts reporting clinical outcomes nearly all date from before the Option B+ era of combined ART (cART) recommended for all HIV-infected pregnant women. It is unknown whether the increased disease burden of HEU has been diminished by cART. The preponderance of current evidence suggests that the immune systems of HEU children differ in fundamental ways from HU children. However, the clinical importance of these differences is far less clear. To date, none of these immune alterations have been correlated with clinical outcome. The goal of HEUICS is to determine the relationship between HEU status, maternal/infant risk factors and infant clinical outcomes. We also propose to investigate 3 plausible determinants of altered neonatal and early infant immune dysfunction: maternal viral load, cART exposure type and duration, and fetal exposure to maternal immune activation (IA). Our overall goals are to determine: 1) if a meaningful increase in morbidity and/or mortality exists among HEU infants by 6 months of age despite maternal cART during pregnancy, compared with a similar cohort of HIV-unexposed children (SA1); 2) the degree of IA among pregnant women on cART and its association with infant clinical outcome (SA2) and; 3) the association of IA with normal and abnormal responses of the early infant cellular immune system (SA3). We will enroll 1500 pregnant women with and without HIV and follow the mother-infant pairs through 6 months of age, the window during which infant mortality rates are highest. Maternal IA status will be determined during the last trimester or at delivery. Outcomes assessed will be morbidity (infections, sick visits, hospitalization frequency and length) and mortality. Predictive clinical and laboratory risk factors of increased HEU morbidity and mortality will be sought. We will enroll a nested subset of 130 mothers with high and low levels of IA and compare infant innate immune makers that correlate with maternal IA. Infants will be followed through 6 months of age to assess their early infant disease burden. 1! !